Yoshiki and Hikaru, boys who grew up together in a certain village.

Yoshiki decides to live with the "something" that has changed into Hikaru.
However, signs of change begin to appear not only in the surroundings but also in Hikaru.
The two, realizing that things can't continue like this, continue their investigation to find out the identity of "something."
The history of "Kubitachi," the true identity of "Nounuki-sama," and what on earth is "Indo's Sin."
When all of that was revealed, the decision that Hikaru, who was "something," made was─.

The book also includes a special short story about “living together.”